This study is designed to investigate the effects of exercise training and detraining (cessation of exercise) on the mechanical performance of the male rat heart and to correlate it with structural changes. Although exercise training of appropriate intensity and frequency has been shown to induce cardiac hypertrophy relatively little is known about the function of the physiologically hypertrophied myocardium. A particular deficiency exists concerning the effects of deconditioning on cardiac structures and function. Using both isolated perfused hearts and isolated myocardium, this study will follow changes in left ventricular output, coronary flow, myocardial oxygen uptake, contractility indices, and cardiac efficiency in response to alterations in preload and afterload; and in response to norepinephrine and hypoxia. These results will be correlated with changes in fiber size and heart weight index. The hearts of 12 week exercise-trained rats will be studied upon completion of training, at 6 weeks after exercise has been discontinued, and after 12 weeks of retraining. In order to assess the influence of age on the cardiac training-detraining response these studies will be performed in three populations of male rats: 2,7, and 18 months of age at the time of entry into the protocol. Providing appropriate care and modification is used in extrapolating such data, the results should give some indication as to how to proceed with human studies dealing with the effects of exercise training on the heart.